The LM393 dual comparator and LM358 dual operational amplifier (op-amp) are widely used in electronic circuits, often in similar packages (e.g., 8-pin DIP). While their pinouts may appear interchangeable, their functional roles and internal designs differ significantly. This article explores their key differences, applications, and whether they can substitute for one another.

1. Functional Differences

• LM393 (Comparator):
  Designed for open-loop voltage comparison, the LM393 compares two input voltages and outputs a digital signal (high/low) based on which input is larger. It operates in saturation (non-linear) mode and is optimized for fast switching.

• Typical Use Cases: Threshold detection, window comparators, motor control, and analog-to-digital conversion.

• LM358 (Op-Amp):
  Intended for linear amplification with negative feedback. The LM358 amplifies small differential signals while maintaining linearity and stability.

• Typical Use Cases: Active filters, signal conditioning, integrators, and voltage followers.

2. Internal Structure and Output Configuration

• LM393:

• Open-Collector Output: Requires an external pull-up resistor to drive a high logic level. This allows flexible interfacing with different voltage levels (e.g., 5V logic with a 12V supply).

• Push-Pull Output Stage (Absent): Can only sink current, not source it (without a pull-up).

• No Phase Compensation: Optimized for speed, not stability in closed-loop configurations.

• LM358:

• Push-Pull Output: Can both source and sink current (up to ~40 mA) without external components.

• Internal Compensation: Includes phase compensation for stable operation in closed-loop feedback circuits.

• Class-AB Output Stage: Enables rail-to-rail output swing (close to supply rails).

3. Key Performance Parameters

4. Critical Differences Affecting Interchangeability

• Output Behavior:
  The LM393’s open-collector output requires a pull-up resistor for logic-high states, whereas the LM358’s push-pull output does not. Substituting one for the other necessitates circuit modifications (e.g., adding/removing pull-ups).

• Speed:
  The LM393’s propagation delay (~1.3 μs) makes it faster for switching applications. The LM358, with its slower slew rate (0.3 V/μs), may introduce lag in comparator roles.

• Stability:
  The LM358’s internal compensation prevents oscillation in feedback loops. The LM393 lacks this, making it unstable in op-amp configurations.

• Saturation Recovery:
  Op-amps like the LM358 suffer from prolonged recovery time when driven into saturation (e.g., used as comparators), leading to delays in dynamic applications.

5. When Can They Be Substituted?

• LM358 as a Comparator:
  Possible in low-speed, non-critical applications (e.g., slow voltage monitoring). Key considerations:

• Add hysteresis to avoid noise-induced oscillations.

• Accept slower response times (~microseconds to milliseconds).

• LM393 as an Op-Amp:
  Generally not recommended due to lack of phase compensation. However, in rare cases with external compensation and feedback, it might function in low-gain applications.

6. Design Considerations for Substitution

1. Pull-Up Resistor: Mandatory for LM393 when replacing LM358 in comparator circuits.

2. Hysteresis: Essential for noise immunity if using LM358 as a comparator.

3. Current Sourcing: LM393 cannot source current without a pull-up; LM358 can.

4. Stability: Avoid using LM393 in closed-loop configurations without compensation.

Conclusion: Are They Interchangeable?

While the LM393 and LM358 share physical packaging and voltage ranges, they are not directly interchangeable without modifications. The LM358 can occasionally replace the LM393 in low-speed comparator roles with added hysteresis, but the LM393 is ill-suited for op-amp circuits. Engineers should prioritize each component’s intended role for optimal performance.

Final Recommendation: Use the LM393 for fast, open-loop switching and the LM358 for linear, feedback-based amplification. Substitution should only occur in non-critical applications after careful analysis.